Generators normally comprise a rotor and a stator, the rotor being arranged rotatably relative to the stator with a small air gap there between. It is necessary to achieve and maintain a high precision in the air gap between the rotor and the stator since the air gap, also for very large generators, has to be quite small, and generally only a few millimetres. To this end the rotor and the stator have previously been designed very rigid and heavy.
Some manufacturers have attempted to overcome these design challenges by mounting roller elements to the stator. The roller elements help maintain the air gap so that the rotor and stator do not need to be as rigid and heavy. Despite these solutions, there remains significant room for improvement.